This invention relates to the catalyzed bromination of hydroxyaromatic compounds, and more particularly to the use of elemental bromine as the brominating agent in such reactions.
Monocyclic dihydroxyaromatic compounds such as hydroquinone and dihydroxybiphenyls such as 4,4′-dihydroxybiphenyl (“biphenol”) have numerous uses in the chemical industry. For example, both compounds can be used in polymer preparation, notably in the preparation of polycarbonates, polysulfones and polyimides, especially polyetherimides.
There are various methods for the preparation of hydroquinone and biphenol. As examples of such methods, each compound can be prepared from p-bromophenol: hydroquinone by hydrolysis, and biphenol by reductive coupling in the presence of a noble metal catalyst, a base, and a reducing agent.
Brominated hydroxyaromatic compounds, as exemplified by p-bromophenol, are typically prepared by reaction of the precursor hydroxyaromatic compound with elemental bromine as the brominating agent in various solvents. However, this process is inefficient in its utilization of bromine because for every mole of bromoaromatic compound produced there is one mole of HBr generated, leading to only 50% conversion of elemental bromine to brominated products. Therefore, improved conversion of bromine to brominated products is desirable and has been pursued.
Several methods have been developed to increase the yield of brominated products with respect to elemental bromine. For example, hydrogen peroxide has been used in combination with the HBr produced during a typical bromination reaction. The use of peroxide facilitates the oxidation of HBr to Br2 and therefore increases the yield of brominated products with respect to the initial amount of bromine charged to the reaction. Other methods include the use of a combination of various catalysts with a peroxide to facilitate the oxidation of HBr to bromine.
While these approaches provide additional efficiencies relative to other known methods, improved methods continue to be sought. In particular, it would be advantageous to conduct bromination reactions using elemental bromine as the brominating agent without producing HBr for several reasons. First, a higher percentage of valuable brominated products, such as p-bromophenol, can be obtained from a given amount of bromine. In addition, the ability to conduct such reactions without forming HBr allows for simplified downstream processing operations, such as eliminating the need for an HBr scrubbing unit and providing more flexibilty in material of construction for the majority of the downstream equipment.